Source of sound approaching the listener


Ladies and gentlemen based on that information that i've explained to you guys is it a positive sign or is it a negative sign over there remember a we are now dealing with the source okay we are now dealing with the source okay so obviously ladies and gentlemen obviously um you know the the source is approaching a the observer okay the source is approaching the observer and it's not moving away okay so let's make a choice ladies and gentlemen let's make a choice over there you say a we used a positive sign because it suits that scenario over there okay so because i've got a positive sign at the top ladies and gentlemen it will be a negative sign at the bottom so this is a a 20 meters per second.

Okay so ladies and gentlemen it's more like now if these are approaching with each other then that's where you have a positive value there at the top then the opposite is the truth in the denominator side okay very very much important ladies and gentlemen that you understand um this kind of information okay so the velocity of the source in this case is approaching the listener okay so if it's approaching the listener, then we know at the top is it's positive then at the bottom it becomes a negative and this is multiplied by the frequency of the source okay and the frequency of the source is given to us as 450 okay very very much important ladies and gentlemen.

So all that you do guys all that you do is just to punch all of this on a calculator so let's see you just uh just do a fraction pattern then in brackets you say 340 then you add zero in brackets then at the bottom open is 340 you close your minus into it's 20 then you multiply 458 which is equals to 486.62 which can be rounded off to two decimal places as four eight six point eight let me see six six three ladies and gentlemen what is the si you need for what i'm calculating remember i'm calculating the frequency so frequency is measured in Hz.

Okay very very much important ladies and gentlemen that you write the s i unit otherwise if you just write that number then we don't know what number it is we're not sure if this is the final answer or this is just part of this and you forgot to multiply by 458 we just don't know exactly what it is but when you write as we know you've reached your final stage of calculating the frequency of the listener okay very very much important ladies and gentlemen.

So now a this is where you score your marks remember this question is allocated for max okay so the reason why i like physical sciences ladies and gentlemen they just give you a mark for copying the formula from the formula sheet very very much important ladies and gentlemen they give you a mark for coping okay so again they give you a mark for correct substitution very very much important ladies and gentlemen the moment you identify the correct formula make sure that you substitute correctly because by just substituting correctly ladies and gentlemen you score some marks that's how easy physical sciences is you score marks for copying what copying ladies and gentlemen not just coping a friend next to you or classmate next to you but for copying the correct formula from the formula sheet.

Okay and next they give you some marks for substituting correctly very very much important ladies and gentlemen and your last mark obviously comes from your final answer with the correct si units ladies and gentlemen it's very very much important that as soon as you write your final answer and you don't forget to write the si unit okay very very much important ladies and gentlemen so now um let's move to the next question i assume um he fully understood how to answer this question.

By the way it was one of the easiest questions because the question was straightforward they just wanted you to calculate um you know the the frequency of the listener so wasn't that much complicated so ladies and gentlemen let's now um check out this question it's 6.3 so the question says how will the observed frequency change as the train passes and moves away from the observer write down only increase decrease or remain the same okay one thing for sure guys because there's a moving object over there then it means a remains the same will not form part of the answer okay then you just need to choose from those two.

Very very much important ladies and gentlemen you just need to understand what they are talking about are they talking about the the frequency of the train or are they talking about the frequency of the listener this is the most important part ladies and gentlemen so in this case they are talking about the observed frequency the frequency of the listener the frequency of the detector.

Okay so ladies and gentlemen as the train passes and moves away from the observer so they're just asking ladies and gentlemen as this train is at that stage as it passes and moving away will the sound head by this guy increase or decrease we've already confirmed that remain remaining the same or remains the same one from part of the answer because that object is moving okay so obviously this entitlement the sound is going to inherit to decrease the frequency is going to decrease what is it that is increasing is the wavelength just in case if a question is asked differently the wavelength is the one which is increasing but the frequency is decreasing so in answering the question you just say decrease okay or you can just write it exactly as it is from the multiple choice.

So you just write decreases and remember this question was allocated just one mark okay so this is how you score a this is how you score a mark for this question so ladies and gentlemen let's now quickly sort out the last question it's not that much difficult easy and straightforward and that's how it is how will the frequency observed by the train driver compare to that of the sound waves emitted by the whistle write down only greater than equal to or less than ladies and gentlemen give a reason for the answer very very much important ladies and gentlemen that you understand exactly what they are talking about over there.

So now they are talking about the train driver ladies and gentlemen assuming that this is a steering wheel and then even though there might not be a steering wheel which looks like the one in the car but this is our grandfather that they're talking about ladies and gentlemen who's driving the train which is making a noise remember this train is is blowing a whistle okay so ladies and gentlemen as this train is moving and passing that gentleman over there ladies and gentlemen 

how does the the sound head by our grandfather over there a compare how does the a you know a sound waves emit head by the trail compare a with what our grandfather is hearing over there ladies and gentlemen remember our grandfather is within that train and as it moves even the sound is moving with the train so that means because he's moving with the sound i mean with the train because the velocity of our grandfather a is the same as the velocity of the train ladies and gentlemen 

because there's no relative motion between the sound and a you know a you know they between the the source or the observer a there's no relative motion between the two ladies and gentlemen that's what i'm trying to say to say between the source which is a that sound emitted and again the observer or the the train the train driver there's no relative motion between the two it's not like the train is moving faster and leaving the observer of the train driver behind they are both moving at the same velocity and for that reason it means a you know the answer is equals to it it cannot be greater than it cannot be less than but it should be equals to okay and i've already given the reasons 

if you understood when i was trying to indicate i mean to interpret that statement because it could be a bit tricky ladies and gentlemen so you just need to be patient with you with yourself try to think uh deep about the question take your time and come up with the correct answer so in answering the question ladies and gentlemen just saying equals two okay so you give everything you can say train has a not necessarily just the train but you can just see the train the train driver has the same a speed the whistle this is an answer that you can provide as a reason to equals two.

And again ladies and gentlemen you can still say no or there's no relative motion between a let's say between source or train driver now the observer is the observer is actually the train driver let me just say train driver which is a the observer so the train driver is the observer and the source so ladies and gentlemen and you can answer this question however you feel that it's easy and then from there by so saying we are going to give you a mark over there and one mark for a providing a reason so this question is then allocated to max okay very very much important ladies and gentlemen that you fully understand 

what i've been explaining to you so guys i thank you very much for joining i hope you enjoyed this lesson as much as i did and guys if you have watched this lesson until this far please do make sure to give this video a like ladies and gentlemen please make sure to like the video and again if necessary drop me a comment just to give me feedback if the lesson was clear in that way i'm able to then up my game if necessary otherwise i'm able to confidently do more of this kind of content because it doesn't save a purpose if i continue creating content or solving these questions whereas i'm confusing you guys so ladies and gentlemen thanks for joining um my name is Desmond and i'm out.

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